The prior art of air separation to produce oxygen and nitrogen through cryogenic distillative separations is well developed. Initially, the industrial gas industry sought to maximize the production of oxygen and to recover it at high purities. When developing oxygen recovery distillation systems, the resulting nitrogen by-product had typically been considered a waste stream having low nitrogen purity. In recent years, nitrogen as a product has developed a commercial significance. Nitrogen is typically utilized as an inerting medium. As such a medium, it was typically required in relatively small amounts or volumes. However, with the depletion of petroleum reservoirs, the need for various forms of enhanced recovery, such as secondary and tertiary recovery techniques for petroleum has been appreciated. Nitrogen has recently been utilized as an inert gas medium which may be utilized to assist in the production of petroleum reservoirs. This use of nitrogen, unlike most uses of nitrogen in the past, requires large volumes of nitrogen at very low per unit costs and at pressures significantly higher than most past uses so as to be readily adaptable to the high pressure conditions of enhanced petroleum recovery operations. Therefore, a need presently exists for a process to produce large volumes of nitrogen at relatively high pressure at a relatively low per unit cost for uses, such as in enhanced petroleum recovery operations.
The art of cryogenic air separation has typically used one or more pressure stages in a distillation column to effect the separation of nitrogen and oxygen from air. For instance, in U.S. Pat. No. 2,089,543, two separate flowschemes for the separation of air into oxygen and nitrogen are shown. In FIG. 1 of that patent, a single stage distillation is illustrated. In FIG. 2 of that patent, a two pressure stage distillation column is shown having an interstage reboiler condenser F and a side reboiler condenser H. The side reboiler produces nitrogen reflux in heat exchange against oxygen liquid from the low pressure stage G of the distillation column. It is apparent that the side reboiler in the oxygen flow passages is at the same pressure as the low pressure stage of the distillation column as is apparent from open lines 37, 38 and 39. In addition, the reboiler condenser F and side reboiler condenser H of that patent operate in series oxygen flow for the heat exchange function and production of nitrogen reflux. By maintaining the pressure of side reboiler H at the pressure of the low pressure stage G, the feed air compression to the high pressure stage E of the distillation column of this patent remains relatively high.
U.S. Pat. No. 4,464,191 discloses a three column cryogenic distillative separation of air into oxygen and two pressure ranges of nitrogen. Side column 5 rectifies an oxygen enriched fluid from low pressure stage 2 into an oxygen product and a further purified nitrogen stream which is returned to the low pressure stage 2. The use of nitrogen from the high pressure stage 1 introduced into the overhead condenser 9 and eventually recovered reduces the overall nitrogen recovery that is possible from a cryogenic distillative separation.
Other high volume nitrogen recovery cryogenic distillative separation systems are disclosed in U.S. Pat. Nos. 4,222,756, 4,453,957 and 4,464,188, none of which utilize a side reboiler to increase the efficiency of nitrogen product recovery.